Flat panel displays (FPDs) are well known devices commonly used in industrial and consumer devices. The most common FPDs are the Liquid Crystal Displays (LCDs) and the Plasma Display Panels (PDPs). The FPDs have relatively small footprints as compared to Cathode Ray Tube (CRT) displays. They consume less energy. The images produced are not distorted at the edges. FPDs are currently used in almost every field of technology that requires a graphic display. They are fast replacing CRT monitors as the de facto output peripheral for personal computers. FPDs are also displacing the conventional CRT television sets.
One of the important elements of the FPDs is the glass substrates which are used to make the viewable screen of the displays. The quality requirements of such glass substrates are very high. They have to be of high clarity, high degree of flatness, uniform thickness and free from contaminants. These glass substrates are now manufactured in large pieces measuring about 890 mm by 680 mm with a thickness of about 0.7 mm. They are later cut into smaller pieces for their various applications.
FPD manufacturers often rely on service providers to perform the cleaning task and to deliver the ready-to-use glass substrates according to their requirements to reduce their overhead costs. The typical standard of cleanliness that is required of glass substrates of the size 890 mm by 680 mm is approximately 15,000 particles of size 0.3 micron.
In a glass substrate cleaning plant, the manufactured glass substrates are delivered in packages of 10-12, held together by four slotted angles made from urethane foam in order to protect each glass substrate from another. The surface of these glass substrates at this stage is highly contaminated. The contaminants range from small general particulate matter like dust to other organic compounds and protein matter. Some of these organic compounds and protein matter can be attributed to human handling and can include natural oils from the skin or simply put “fingerprints”.
The washing or cleaning of glass substrates in the manufacturing of FPDs is crucial. It ensures that the surfaces of the glass substrates are free from contaminants. This is necessary for the proper coating requirements according to the different types of FPDs. The importance of cleaning the glass substrates is basically to ensure the clarity of the images produced when the FPDs are finally assembled. Contaminants present on the surfaces of the glass substrates prevent proper coating and causes distortion.
Some of the possible approaches or methods used in cleaning glass substrates are: high temperature de-ionized water, the Radiance Process®, High Power Ultrasonic cleaning and Plasma cleaning.
The currently most practiced method of cleaning glass substrates is to first wash them with high temperature de-ionized (DI) water, followed by vacuum drying. The glass substrates are then conveyed directly into a clean room environment for packaging. This method though somewhat effective, consumes several million cubic meters of DI water for a typical production process. The use of vacuum drying also increases costs, as power requirements are also very high. Studies have indicated that a plant cleaning 5000 FPD parts a day would require about 1.44 cubic meters of DI water per FPD part. This washing method also becomes less effective when the particulate size approaches 0.3 microns.
The Radiance Process® makes use of a laser and a continuous laminar flow of an inert gas to remove contaminants. The laser breaks the bonds holding the contaminants to the glass substrate and the inert gas carries the contaminants away. Recent demonstrations have shown that the total particulate counts were reduced dramatically. However, the Radiance Process® is highly dependent on maintaining the laminar flow of inert gases over the glass substrate surface. The equipment to provide and carry out this Radiance Process® is presently not available at a low cost.
Another method used is High Power Ultrasonic cleaning. The technology itself is not new. Ultrasonic cleaning has already been used with Printed Circuit Boards. It has been applied here in the cleaning of glass substrates as well. However, the method is far from perfect. During the ultrasonic cleaning process, cavitation of the cleaning fluid results in formation of small bubbles impacting onto the surface of the glass substrates. This sometimes causes micro-fractures on the glass surface and renders the glass substrate unusable.
Plasma Cleaning technology is performed in high vacuum systems by exposing the parts to be cleaned to energized plasmas. Though effective, plasma systems require high sources of energy to energize the plasmas and to provide the high vacuum environments. Invariably, such a system might be too expensive to use merely on account of cleaning the surfaces of a glass substrate.
There is thus a need for a novel, effective and economical method and apparatus for removing of contaminants from surfaces of glass substrates.